The present invention generally relates to focus control methods and optical disk units, and more particularly to a focus control method which corrects an offset in a focus servo system and to an optical disk unit which employs such a focus control method.
FIG. 1 is a system block diagram showing an example of a conventional optical disk unit. In FIG. 1, a laser beam emitted from a laser diode (LD) 101 of an optical head 100 is irradiated on an optical disk 103 which is rotated by a spindle motor 103. The reflected laser beam from the optical disk 103 is converted into an electrical signal by a photodetector 104 within the optical head 100. A laser diode control circuit 105 controls the laser diode 101 based on a gate signal which is for a write/erase/read mode and is received from a host unit (not shown), so that the laser diode 101 emits the laser beam with an optimum light emitting power for each mode.
A reproducing circuit 106 separates the frequency band of the electrical signal which is received from the photodetector 104 of the optical head 100, and a high-frequency component signal from the reproducing circuit 106 is supplied to a radio frequency (RF) signal generating circuit (not shown) which reproduces data. On the other hand, a low-frequency component signal from the reproducing circuit 106 is supplied to a tracking servo system 107 and a focus servo system. The tracking servo system 107 generates a tracking error signal based on the low-frequency component signal and supplies this tracking error signal to the optical head 100 so as to carry out a tracking servo. On the other hand, the focus servo system 108 generates a focus error signal based on the low-frequency component signal and supplies this focus error signal to the optical head 100 so as to carry out a focus servo.
However, the conventional optical disk unit had problems (1) through (3) which are described hereunder.
(1) In the case of the optical disk unit in which the light emitting power of the laser diode 101 is switched depending on the write/erase/read mode, a so-called mode hop occurs whereby the oscillation wavelength of the laser diode 101 switches depending on the mode, and there was a problem in that the focal point moves. The amount or extent of mode hop differs among each individual laser diode 101, and also changes depending on the temperature.
FIGS. 2(A) through 2(C) are timing charts for explaining a distortion in the focus servo due to the switching of the light emitting power. In FIG. 2(A) shows a normal focus error signal FES, FIG. 2(B) shows the focus error signal FES when the focus servo is distorted depending on the mode, and FIG. 2(C) shows a write gate signal WG. It may be seen from FIGS. 2(A) through 2(C) that a focus error is generated when the light emitting power of the laser diode 101 is switched and that the focal point is returned to the in-focus position by the focus servo. Such a focus error is generated immediately after the start of the write and immediately after the end of the write when the light emitting power of the laser diode 101 is switched. Particularly the focus error which is generated immediately after the start of the write greatly affects the write performance, and easily becomes the cause of a reproduction error during the read. In addition, there was also a problem in that the focus servo is distorted at a constant period and noise is generated thereby.
In FIG. 2(C), a high-level period of the write gate signal WG indicates a write/erase mode, and a low-level period of the write gate signal WG indicates a read mode. In other words, in the particular case shown in FIGS. 2(A) through 2(C), the write/erasure of data is carried out with respect to the optical disk 103 during the high-level period of the write gate signal WG, and an identification (ID) signal is read during the low-level period of the write gate signal WG.
(2) In a fixed optical system of the optical disk unit, the positions of the photodetector and the like are adjusted so that the focus servo can be made to the optimum in-focus position by the fixed optical system alone. However, there was a problem in that the focus servo may be made to a position different from the adjusted in-focus position due to the mounting error that occurs when the fixed optical system is mounted on a base of the optical disk unit and due to an offset of a circuit which processes an output signal of the fixed optical system.
(3) Furthermore, when a temperature change occurs, the positions and angles of optical parts deviate from their original correct positions depending on the difference between the coefficients of thermal expansion of the optical parts and the fixed parts and depending on the temperature characteristic of an adhesive agent or the like that is used to fix the optical parts. For this reason, an offset is generated in the focusing direction due to the temperature change, and there was a problem in that the focus servo may be made to a position deviated from the optimum in-focus position.
If any one of the problems (1) through (3) described above becomes the cause and the focus servo is not made to the optimum in-focus position, a write error and/or a read error occurs.
With respect to the problems (2) and (3), that is, with respect to the problem of the focus servo being made to a position deviated from the optimum in-focus position, it is possible to reduce the undesirable effects to a certain extent by making an adjustment so that the amplitude of the tracking error signal becomes a maximum. For example, such a method is proposed in Japanese Laid-Open Patent Applications No.62-128027, No.62-141644, No.62-222438 and No.2-230516. However, with respect to the problem (2), it is necessary to make one adjustment during the time from a time when the optical disk is loaded into the optical disk unit to a time when the optical disk unit assumes a ready state. In addition, with respect to the problem (3), it is necessary to make the adjustment every time a temperature change occurs. For these reasons, there still were problems in that it takes time to make the necessary adjustments and that the performance of the optical disk unit deteriorates.